1. Field of the Invention
This invention relates generally to magnetic float polishing and, more specifically, to a methodology for obtaining superior finish on ceramic balls using both mechanical and chemo-mechanical action in magnetic float polishing. The invention also encompasses chemo-mechanical polishing agents used in the processes.
2. Background
The development of high-performance ceramics (or advanced ceramics) is stimulating major advances in a large spectrum of industries including machine tools, electronics, manufacturing engineering, and chemical and metallurgical processing. Alumina (Al.sub.2 O.sub.3), zirconia (ZrO.sub.2), silicon carbide (SiC) and silicon nitride (Si.sub.3 N.sub.4) are the most important advanced ceramic materials among high-performance ceramics with Si.sub.3 N.sub.4 being the most promising material in this category for advanced structural bearing applications. Ceramic bearings offer significant improvements in performance and durability for a wide variety of applications ranging from inertial guidance systems to precision gimbals to turbine engine exhaust nozzle actuators and submarine pumps. Hybrid bearings with silicon nitride balls have made 100,000 rpm a possibility for high speed machine tool spindles. This is principally due to higher rigidity and greater precision associated with these bearings.
A critical factor affecting the performance and reliability of ceramics for bearing applications is the quality of the resulting surface by polishing. In fact, non-uniform grinding and polishing techniques have been identified recently by the U.S. Department of Defense as a principal barrier to the greater use of ceramics. It is well known that ceramics are extremely sensitive to surface defects resulting from grinding and polishing processes owing to their high hardness and inherent brittleness. Since fatigue failure of ceramics is driven by surface imperfections, it is paramount that the quality and finish of the ceramic bearing elements be as smooth as possible with minimal defects so that reliability in service and improvements in the performance of the bearings can be achieved.
Due to their hardness and brittleness, most advanced ceramic materials are extremely difficult to shape and finish. Unlike the situation with metals, plastic deformation is not the preferred mode of material removal. Instead, material removal is by brittle fracture. Consequently, with conventional grinding and polishing techniques surface damage is inherently present on the workpiece in the form of pits and scratches, and subsurface damage in the form of lateral and radial cracks. These defects affect the performance and reliability of the products in service.
Conventional polishing of ceramic balls generally uses diamond abrasive, high load, and low polishing speeds (maximum of a few hundred rpm). This is basically the same technology that is used for finishing metal balls extended to the finishing of ceramic and glass balls. This is in spite of the fact that different mechanisms are involved in the material removal processes due to difference in material characteristics and their response to polishing conditions. Considerable time is expended (estimates vary from some 4-6 weeks to 12-16 weeks depending on the size of the balls, the quality requirements, and the manufacturing technology practices) to finish a batch of ceramic balls. The long processing times and the use of expensive diamond abrasive result in high processing costs. Application of diamond abrasive under high loads in conventional polishing often results in deep scratches, pits, and microcracks on the surface of the polished balls. Consequently, performance in service and reliability are major concerns with the conventional polishing of ceramics. To address these problems, need arises for an alternate technique that minimizes the defects and other disadvantages associated with conventional material removal processes.